Drs. Eric Hoffman and Dietrich Stephan (whose primary interests are focused on muscle disease and leukemia) are PI and Co-PI on the Program's Expression Array Core and have extensive experience with all aspects of array use and data analysis. They have established collaborations with Dr. Michael Bitmer, Dr. Yidony Chen and the entire NHGRI array community. As a post-doctoral fellow in the laboratory of Dr. Jeffery Trent (member of this Program's External Advisory Committee), and a pioneer in DNA microarray technology, Dr. Stephan developed several alternative approaches to label total RNA for efficient signal detection under varying circumstances which have become the standard protocol used at NHGRI and has become a quite robust and reliable system for detecting signals over several orders of magnitude. In addition to building and using 7K expression arrays, Dr. Stephan was the first at NHGRI to develop genomic DNA microarray systems. Dr. Hoffman's laboratory is particularly interested in systematic assessments of the sensitivity and specificity of the Affymetrix vs. cDNA array approaches, and shows preliminary data pointing out the importance of redundant measurements, and correlative studies. Indeed, the proposed systematic comparison of the Affymetrix and cDNA array experimental platforms should be the first of this type of quality control of expression array data, and should prove highly valuable to both Program investigations and the research community at large.